System and method for bandwidth management in ethernet-based fiber optic TDMA networks

ABSTRACT

A system and method for management of bandwidth in a fiber optic, ethernet-based, TDMA communications system. A request/grant process is used to control the use of upstream bandwidth. A sense of time must therefore be shared by a headend and remote end-user devices. The invention provides for a gigabit media-independent interface in a media access controller to detect start-of-frame delimiters in incoming data. This allows for synchronization of a headend and end-user devices. The invention also allows for phase locking a transmit bit rate, at a headend, to the headend&#39;s clock. Transmitted data can the be used downstream to derive a local clock. Synchronization can also be maintained by the use of synchronization bytes in MPEG frames and/or variable length frames. Efficient bandwidth usage can also be facilitated by the use of maximum data units in allocating bandwidth in unsolicited grants, and by allowing flexible fragmentation and/or prioritization of internet protocol (IP) packets.

[0001] This application claims priority to U.S. Provisional PatentApplication No. 60/302,674, filed Jul. 5, 2001, and incorporated hereinby reference in its entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] The following United States and PCT utility patent applicationshave a common assignee and contain some common disclosure:

[0003] “System for Communications in Ethernet-Based Fiber Optic TDMANetworks,” U.S. Application Serial No. TBD (Attorney Docket No.1875.1440001 :BP 1909), by Gummalla et al., filed concurrently herewith,incorporated herein by reference;

[0004] “System for Spectrum Allocation in Ethernet-Based Fiber OpticTDMA Networks,” U.S. Application Serial No. TBD (Attorney Docket No.1875.1440002:BP 1909), by Sala et al., filed concurrently herewith,incorporated herein by reference;

[0005] “System, Method, and Computer Program Product for OptimizingVideo Service in Ethernet-Based Fiber Optic TDMA Networks,” U.S.Application Serial No. TBD (Attorney Docket No. 1875.1440004:BP 1909),by Gummalla et al., filed concurrently herewith, incorporated herein byreference; and

[0006] “System, Method, and Computer Program Product for ManagingCommunications in Ethernet-Based Fiber Optic TDMA Networks,” PCTApplication Serial No. TBD (Attorney Docket No. 1875.144PC01 :BP 1909),by Gummalla et al., filed concurrently herewith, incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0007] 1. Field of the Invention

[0008] The invention described herein relates to data networks, and moreparticularly, to the use of ethernet over a fiber optic network.

[0009] 2. Related Art

[0010] One of the current trends in data networking is the use of fiberoptic media. Moreover, use of ethernet technology is a practical choicefor such networks, given that ethernet is well understood and can besupported by available components. The application of ethernet fibertechnology to relatively long distance access networks creates problems,however. Among the unresolved problems is how to share bandwidthefficiently and cost-effectively among multiple users in such anenvironment. A reasonable quality of service for all users is alsodesirable. Hence there is a need for a system, method, and computerprogram product by which bandwidth can be managed in an ethernet-basedfiber access network, and service can be kept affordable anduser-friendly to end users.

SUMMARY OF THE INVENTION

[0011] This invention addresses management of bandwidth and operationalefficiency in a fiber optic, ethernet-based, TDMA communications system.A request/grant process is used to control the use of upstreambandwidth. A sense of time must therefore be shared by a headend andremote end-user devices. The invention provides for a gigabitmedia-independent interface in a media access controller to detectstart-of-frame delimiters in incoming data. This allows forsynchronization of a headend and end-user devices. The invention alsoallows for phase locking a transmit bit rate, at a headend, to theheadend's clock. Transmitted data can the be used downstream to derive alocal clock. Synchronization can also be maintained by the use ofsynchronization bytes in MPEG frames and/or variable length frames.Efficient bandwidth usage can also be facilitated by the use of maximumdata units in allocating bandwidth in unsolicited grants, and byallowing flexible fragmentation and/or prioritization of internetprotocol (IP) packets.

BRIEF DESCRIPTION OF THE FIGURES

[0012]FIG. 1 is a block diagram illustrating a gigabit media-independentinterface (GMII) incorporated into a media access controller (MAC).

[0013]FIG. 2 illustrates a fiber optic network using an active passiveoptical network (PON) architecture.

[0014]FIG. 3 is a block diagram illustrating the use of an optical nodeto accommodate a variety of communications topologies.

[0015]FIG. 4 is a block diagram illustrating an optical node.

[0016]FIG. 5 illustrates the use of an adaptive equalizer to reducenoise in a light source.

[0017]FIG. 6 is a flowchart is a flowchart illustrating the process oftimebase synchronization where a remote maintains synchronization bylocking on to a phase-locked transmit bit stream.

[0018]FIG. 7 is a flowchart illustrating the use of a synchronizationbyte in a Moving Pictures Expert Group (MPEG) frame to synchronize aremote device.

[0019]FIG. 8 is a flowchart illustrating the use of a synchronizationbyte in an variable length packet to synchronize a remote device.

[0020]FIG. 9 is a flowchart illustrating the gating of upstreamtransmissions according to a grant of a headend.

[0021]FIG. 10A and 10B illustrate the relationship between PONarbitration and the 802.3 protocol.

[0022]FIG. 11 illustrates the use of different wavelengths to carrydownstream video data, downstream non-video data, and upstream data.

[0023]FIG. 12 illustrates the concept of spectral slicing.

[0024]FIG. 13 illustrates hybridization of point-to-point and broadcastarchitectures.

[0025]FIG. 14 illustrates subcarrier multiplexing, wherein each user hasits own subcarrier.

[0026]FIG. 15 is a flowchart illustrating the use of a maximum data unit(MDU) in bandwidth allocation.

[0027]FIG. 16 is a flowchart illustrating flexible packet fragmentationbased on available bandwidth.

[0028]FIG. 17 is a flowchart illustrating reallocation of videobandwidth for non-video data.

[0029]FIG. 18 is a flowchart illustrating the bandwidth request andgrant process.

[0030]FIG. 19 is a flowchart illustrating packet transmission based onpriority and bandwidth availability.

[0031]FIG. 20 is a block diagram illustrating the buffering of MPEGframes at an optical node (ON).

[0032]FIG. 21 is a flow chart illustrating proactive video streaming.

DETAILED DESCRIPTION OF THE INVENTION

[0033] A preferred embodiment of the present invention is now describedwith reference to the figures, where like reference numbers indicateidentical or functionally similar elements. Also in the figures, theleft-most digit of each reference number corresponds to the figure inwhich the reference number is first used. While specific configurationsand arrangements are discussed, it should be understood that this isdone for illustrative purposes only. A person skilled in the relevantart will recognize that other configurations and arrangements can beused without departing from the spirit and scope of the invention. Itwill be apparent to a person skilled in the relevant art that thisinvention can also be employed in a variety of other devices andapplications.

[0034] I. Overview

[0035] In the invention described herein, the use of a time divisionmultiple access (TDMA) architecture allows the sharing of bandwidthamong multiple users. The Data Over Cable System Interface Specification(DOCSIS) describes a process through which bandwidth management andother requirements can be achieved in a TDMA setting. The presentinvention provides means for addressing management of bandwidth, costcontrol, quality of service, and operational efficiency in a fiberoptic, ethernet-based, TDMA communications system.

[0036] In general, DOCSIS can be used in communication systems thatinclude a set of remote communications devices connected to a headenddevice, such that the headend is responsible for the management ofcommunications both to and from the remotes. The headend is responsiblefor the distribution of information content to the remotes (theso-called “downstream” direction); in addition, the headend isresponsible for management of communications in the other direction,from the remotes to the headend (the “upstream” direction). Generally,in addition to sending content to remotes, the headend issues downstreammap messages that instruct each remote as to when it can transmitupstream, and what kind of information it can send. In effect, theupstream bandwidth is controlled and allocated by the headend. Any givenremote can transmit upstream only after requesting bandwidth andreceiving a grant of the bandwidth from the headend. In a time divisionmultiple access (TDMA) environment, bandwidth corresponds to one or moreintervals of time. Moreover, the upstream can be organized into a numberof channels, with several remotes assigned to each channel. Thisarrangement allows the headend to manage each upstream communicationschannel. In this manner, upstream communications are managed so as tomaintain order and efficiency and, consequently, an adequate level ofservice.

[0037] In the realm of cable communications, DOCSIS specifies therequirements for interactions between a cable headend and associatedremote cable modems. A cable headend is also known as a cable modemtermination system (CMTS). DOCSIS consists of a group of specificationsthat cover operations support systems, management, and data interfaces,as well as network layer, data link layer, and physical layer transport.Note that DOCSIS does not specify an application layer. The DOCSISspecification includes extensive media access layer and physical (PHY)layer upstream parameter control for robustness and adaptability. DOCSISalso provides link layer security with authentication. This preventstheft of service and some assurance of traffic integrity.

[0038] The current version of DOCSIS (DOCSIS 1.1) uses a request/grantmechanism for allowing remote devices (such as cable modems) to accessupstream bandwidth. DOCSIS 1.1 also allows the provision of differentservices to different parties who may be tied to a single modem. Withrespect to the processing of packets, DOCSIS 1.1 allows segmentation oflarge packets, which simplifies bandwidth allocation. DOCSIS 1.1 alsoallows for the combining of multiple small packets to increasethroughput as necessary. Security features are present through thespecification of 56-bit Data Encryption Standard (DES) encryption anddecryption, to secure the privacy of a connection. DOCSIS 1.1 alsoprovides for payload header suppression, whereby repetitive ethernet/IPheader information can be suppressed for improved bandwidth utilization.DOCSIS 1.1 also supports dynamic channel change. Either or both of thedownstream and upstream channels can be changed on the fly. This allowsfor load balancing of channels, which can improve robustness.

[0039] While the present invention is described herein in the context ofDOCSIS, it should be understood that the systems and methods discussedbelow are also applicable in other contexts as well. Generally, thesesystems and methods are applicable to any fiber access system.

[0040] Note that in the discussion below, techniques are organizedgenerally according to their benefit, i.e., cost control, bandwidthmanagement, user-friendliness, and operational efficiency. This does notnecessarily represent a limitation on the utility or scope of any of thetechniques. A bandwidth management technique may, for example, havebenefits with respect to a system's operational efficiency oruser-friendliness. The categorization below should therefore not beviewed as any sort of limitation of applicability.

[0041] II. Cost Control

[0042] A. Hardware Architecture

[0043] 1. Detection of Reference Point at GMII

[0044] This aspect of the invention allows the use of existing,commercially available ethernet physical layer devices in a fiber opticTDMA network operating under DOCSIS. DOCSIS includes a process whereby aheadend and associated remote devices become synchronized so that theyall share the same sense of time with respect to upstreamcommunications. This synchronization is known in DOCSIS as ranging.Ranging requires that remotes each send a burst of information at a timeprescribed by the headend. The headend must then detect whether theburst arrived too soon or too late, relative to the prescribed arrivaltime. Typically, a specific reference point in the preamble of the burstis used to gauge the burst's arrival. When this point is detected, theburst is considered to have arrived. The start of frame delimiter (SFD)byte in a DOCSIS burst can be used for this purpose.

[0045] Commercially available ethernet physical layer devices, however,do not typically have the functionality that allows detection of aburst's reference point. In this invention, the reference point isobserved not at the PHY device per se, but rather at the interface ofthe physical layer device to the media access controller. Because thedelay through an ethernet physical layer device is nearly constant,however, it is not necessary for this device to detect the referencepoint.

[0046] In an embodiment of the invention, illustrated in FIG. 1, thedetection takes place at a gigabit media-independent interface (GMII)120 of the media access controller (MAC) 110. This can be implemented byhardware in MAC 110 that monitors incoming data from PHY device 130, todetect an SFD byte. The timing reference in the MAC 110 can be used todetermine the time at which the SFD is observed, relative to theexpected time of the SFD's arrival. The GMII and MAC may be placed at anoptical node and/or at a headend device. Note that, in general, a GMIIcan be used to detect any field having a known relationship to the startof a frame, not just an SFD per se, for purposes of detecting a rangingoffset.

[0047] 2. Active PON

[0048] Another issue in the use of a fiber access network is themanagement and allocation of costs in topologies involving relativelylong distances (e.g., 20 kilometers or more). One way to address this isto use an active architecture, instead of a passive optical network(PON) approach.

[0049] Traditionally, in a PON, transmission of information between acentral office and end users, e.g., in their homes, is done through apassive splitter. While this is a workable architecture for relativelyshort distances, longer distances, up to 20 kilometers and beyond, areproblematic. Longer distances require more powerful (and more expensive)light sources.

[0050] Instead, an optical node acting as an aggregation device can beused to handle transmissions over longer distances. An embodiment ofsuch a system is shown in FIG. 2. An upstream feeder channel 205 and adownstream channel 210 are shown, each operating on a differentwavelength. The downstream feeder channel 210 connects a central office(CO) 215 and an optical node (ON) 220. ON 220 serves as an aggregationdevice. A distribution system 225 extends from ON 220 to users 230.Thus, a single expensive laser connects ON 220 and CO 215. Less powerfuland, consequently, cheaper lasers in distribution system 225 connect enduser devices 230 (e.g., modems) to ON 220.

[0051] The relatively high cost of the long distance laser can now beshared among users 230. The distribution path from ON 220 to users 230is relatively cheap, since less power is required for shorter distances.The costs of using a remote device can therefore be lowered by thisapproach.

[0052] Moreover, in an embodiment of the invention, an optical node canaccommodate multiple topologies on the user side. This is illustrated inFIG. 3. A hub 305 sends high bandwidth data, such as digital video, toan ON 310 via a feeder link 315. An end user in a home 320 can receivethe data from ON 310 via a fast ethernet point-to-point (P2P) connection325. Other homes, such as homes 330 a through 330 n, receive data fromON 310 though a shared connection 335.

[0053] An embodiment of an ON is shown in FIG. 4. ON 400 includes agigabit ethernet interface 405 through which ON 400 connects with a hub.Interface 405 is connected to a switch 410. Switch 410 serves toaggregate traffic headed upstream towards a hub, and can be implementedas a multiplexer/demultiplexer. Switch 410 is connected to one or morePON controllers 415 a through 415 m, which arbitrate access and providelink control with respect to end users. Switch 410 and PON controllers415 provide quality of service functions as well, such as the control ofdata flow based on prioritization or based on other categorizations oftraffic. Each PON controller is connected to one or more PHY devices 420a through 420 p. Each PHY device is then connected through a physicalcommunications medium to an end user device (not shown).

[0054] Moreover, in an embodiment of the invention, ON 400 has otherinterfaces (not shown), to support different kinds of traffic, such asvoice, and/or to support circuit emulation.

[0055] B. Operational Efficiency

[0056] 1. Cancellation of Laser Humming

[0057] In any optical access system, the light sources (e.g., lasers)may not operate continually. Rather, they can cycle as necessary betweena powered operational state and an idle state. In the latter, a laser isnot completely powered down. The laser emits light at a low level duringidle, and is said to “hum.” Humming adds noise, affecting thesignal-to-noise ratio (SNR) of other signals in the system.

[0058] This noise can be ameliorated by using an adaptive equalizer. Asis known in the art, an adaptive equalizer can be used to cancel noiseon a communications channel. Such an equalizer can cancel the humming ofa laser during idle, thereby improving the SNR of information-bearingsignals. In an embodiment of the invention, an adaptive equalizer isused as illustrated in FIG. 5. A receiver 500 receives light 505 from alight source, such as a laser. Light 505 is received at an opticaldetector 510. The output of optical detector is fed to adaptiveequalizer 530. Equalizer output 530 produces an equalized output 550.

[0059] 2. Spectral Slicing

[0060] Spectral slicing is a technique by which multiple users can usedifferent frequency bands of the same broadband laser source forcommunication. This is illustrated generally in FIG. 12. Users transmitusing different frequencies 1205. These frequencies represent slices ofa broadband transmission 1210. This technique enables the implementationof point-to-point links in a point-to-multipoint topology. Since eachsubscriber uses a different frequency band, subscribers do not interferewith each other.

[0061] In such a system there is a tradeoff between the number ofsubscribers, the bandwidth of the filters required, and the transmitpower from each subscriber. There is also a cost tradeoff based on the Qof the filters required. A technique is used in which each subscriberunit has multiple light emitting diodes (LEDs). In an embodiment of theinvention, four LEDs are used by subscribers, red, blue, green andyellow. A subscriber can use any one of them for communication with thehub/ON. Since LEDs are very cheap, they will not add significant cost tothe subscriber unit. Each unit uses lower Q filtering (representinglower cost) and, as a result, gets to use higher transmit power. Thereceiver in the ON can split the four different wavelengths usingdevices like Briggs grating and can demultiplex different subscribers ineach wavelength using filters. This enhances the efficiency of bandwidthusage and increases the number of subscribers per port at hub/ON. Inaddition, this reduces the cost of the overall system.

[0062] The split ratio of the PONs can be increased by using signalprocessing techniques. By using forward error correction (FEC), codinggains on the order of 3-6 dB can be achieved. This can easily double orquadruple the number of subscribers on a single PON. A furtherimprovement of 3 dB can be achieved using adaptive equalization, whichcan double the subscribers. Since these signal processing techniques canbe adding at very little additional cost, the overall cost of the systemper subscriber drops significantly.

[0063] 3. Allocation of Functionality Between Hub and Optical Node

[0064] An ON, like any other communications component, has limits as tothe functionality that it can incorporate. Factors such as chip size andpower dissipation must be considered during system design. DOCSIS,however, requires certain functionality at a headend. This includestiming and sequencing functions, such as ranging. DOCSIS also requiresbandwidth allocation processing, such as the generation of map messages.It also requires subscriber service functions, such as authenticationand billing.

[0065] Because all this functionality can be difficult to put in asingle component, a better approach is the dispersal of thefunctionality. In the context of an optical network such as that of FIG.3, some functionality, such as the subscriber service functions, can beplaced in the hub 305. Other functions, such as timing and bandwidthallocation functions, can be placed in the ON 310. This reduces theprocessing burden on any single component, with no loss in overallsystem capability.

[0066] C. Bandwidth Management

[0067] 1. Timebase Synchronization

[0068] TDMA systems require the maintenance of a time base which is usedto determine time slot boundaries, communicate the time base to all theequipment in the system, and chronologically lock equipment to the timebase. The current state of the art is exemplified by the DOCSISspecification. In such systems, the headend generates a time base in theform of a time stamp counter driven by a very precise referenceoscillator. The headend communicates the time base to one or more remotedevices (e.g., cable modems) via periodic synchronization messages.These messages contain the current time stamp counter value. There areseveral problems with such a system. Among them, time stamps must besent relatively often, and the time, as maintained at a remote, candrift slowly so that it can move several counts away from the headend'scount. Recovering from such a variation can take a long period of time.

[0069] One method of maintaining synchronization in a TDMA system is theuse of synchronous rate locking to keep the rates of the time stampcounts at the headend and each remote device locked to each other. Oneembodiment uses ethernet PHY devices at both ends. The transmitting PHYat the headend can be viewed as the master. This method is illustratedin FIG. 6. The method starts at step 605. In step 610, the transmittingPHY's transmit bit rate (i.e., the symbol rate for optical PHY) is phaselocked to the clock used to generate the headend time stamp counter. Instep 620, transmission begins. In step 625, the receiving PHY device atthe remote locks on to the bit rate of the incoming data stream. Fromthis the remote's clock is derived locally, which drives the remote'slocal copy of the time stamp counter (step 630). The method concludes atstep 640. Using this method, synchronization messages need not be sentoften since they are only used to initialize the counter of a remotewhen it joins the network and to periodically check the counter againstthe current value. To initialize, the remote simply loads the first timestamp it receives into its local register. Techniques such as blockcoding or scrambling can be used to control clock jitter. Block codinghas the advantage of maintaining DC balance and can also maintain therequired number of bit transitions. Scrambling techniques can have muchless overhead.

[0070] Other techniques by which time base synchronization can bemaintained in a TDMA optical system include an increased frequency ofsynchronization messages to deal with jitter. This imposes a requirementof the time base generator at the headend to be accurate within 100picoseconds. This also requires the remote to have tight control on itsjitter.

[0071] Another option is to use physical layer in-band synchronizationusing MPEG framing. An MPEG frame has a synchronization byte at thestart of the frame. This byte has a specific predeterminedsynchronization pattern. A remote latches to the periodicsynchronization byte to synchronize to the downstream rate. This processis illustrated in FIG. 7, according to an embodiment of the invention.The process begins with step 710. In step 720, a counter is initialized.This counter is used to count the number of times, in succession, thatthe synchronization pattern is successfully found. In step 730, theremote device searches for the synchronization pattern in incomingtraffic. If the pattern is not found, as determined in step 740,searching continues at step 730. If the pattern is found, processingcontinues at step 750, where the counter is incremented. In step 760, adetermination is made as to whether the counter has reached a thresholdvalue. If not, processing continues at step 770. Here, thesynchronization pattern is sought at a subsequent point in the traffic,a predetermined number of bytes later. For fixed-length MPEG frames, thepattern is sought 188 bytes later. If, in step 780, the synchronizationpattern is found, the counter is incremented in step 750, and theprocess repeats from this point. If no synchronization pattern is foundin step 780, the counter is re-initialized in step 720, and the entireprocess restarts. If, in step 760, the threshold is reached, thisindicates that a sufficient number of synchronization patterns have beenfound in consecutive attempts, and synchronization is attained (step790).

[0072] A similar technique can be implemented with variable lengthpackets with the synchronization byte followed by a pointer to the nextsynchronization byte. This is illustrated in FIG. 8. The process beginswith step 810. In step 820, a counter is initialized. Again, thiscounter is used to count the number of times, in succession, that thesynchronization pattern is successfully found. In step 830, the remotedevice searches for the synchronization pattern in incoming traffic. Ifthe pattern is not found, as determined in step 840, searching continuesat step 830. If the pattern is found, processing continues at step 850,where the counter is incremented. In step 860, a determination is madeas to whether the counter has reached a threshold value. If not,processing continues at step 865. Here, a pointer is read, where thepointer is found after the last synchronization pattern. The pointerindicates the location, in the incoming traffic, of the nextsynchronization pattern. In step 870, the next synchronization patternis sought at the indicated point in the traffic. If, in step 880, thesynchronization pattern is found, the counter is incremented in step850, and the process repeats from this point. If no synchronizationpattern is found in step 880, the counter is re-initialized in step 820,and the entire process restarts. If, in step 860, the threshold isreached, this indicates that a sufficient number of synchronizationpatterns have been found in consecutive attempts, and synchronization isattained (step 890).

[0073] 2. Wavelength Allocation, Video and Data

[0074] In the communications systems described herein, bandwidthlimitations can be problematic. Given one gigabit per second ofdownstream bandwidth, for example, 600 megabits could be required fordigital video, leaving only 400 megabits for other data traffic.Typically, video and data signals share the bandwidth through amultiplexing arrangement.

[0075] An alternative is to allocate different wavelengths to differentrequirements. For example, one wavelength could be allocated todownstream digital video, while another wavelength would be allocated todownstream non-video data. A third could be allocated to upstream data.This increases the available bandwidth for each requirement, andrepresents a way to upgrade a traditional PON architecture in light ofthe need for greater capacity. This is illustrated in FIG. 11. Here, acentral office 1110 is in communication with optical node 1120.Downstream digital video is carried on channel 1130, operating at awavelength λ₁. Downstream data (non-video) is carried on channel 1140,operating on a wavelength λ₂. Channel 1150 is used for upstreamcommunications on a wavelength λ₃. Such an arrangement serves toincrease bandwidth between central office 1110 and a set of users 1160.

[0076] Note that a video transmission from central office 1110 can be abroadcast, so that multiple optical nodes may receive the λ₁transmission. Allocation of wavelengths for downstream non-videotransmissions (in FIG. 11, wavelength λ₂) and upstream transmissions(wavelength λ₃), however, is done per optical node.

[0077] 3. Hybrid PON: Broadcast Downstream, Point-to-Point Upstream

[0078] Another architectural solution to the bandwidth constraint issueis to hybridize broadcast and point-to-point concepts. This isillustrated in FIG. 13. A central office (CO) 1305 broadcasts downstreamto all end users, including a remote device 1310, shown here as customerpremises equipment (CPE). The broadcast takes place using a singlewavelength, λ_(d), and passes through a series of couplers, includingcouplers 1315 and 1320.

[0079] Upstream transmissions take place over multiple wavelengths, oneper user, shown here as λ_(u1) through λ_(un). Hence the upstream is apoint-to-point architecture using wavelength division multiplexing(WDM).

[0080] Here, the need for a high-powered laser is limited to the CO1305, as is the need for wavelength detection functionality. Remotedevices, such as CPE 1310, require a high bandwidth receiver (e.g.,gigabit), but can operate with a lower bandwidth transmitter (e.g.,10/100 megabit).

[0081] 4. Subcarrier Multiplexing

[0082] Where upstream bandwidth is problematic, each user can beassigned his or her own frequency, such that all user frequencies areassociated with a single narrowly defined wavelength range. Frequenciescan be offset, for example, by 100 MHz in an embodiment of theinvention. This allows autonomous communication for each user, withoutinterference. This is illustrated in FIG. 14. Here, a central office1410 is in communication with users 1461, 1462, and 1463. Each of thesethree respective users can transmit to central office 1410 usingsubcarriers 1451, 1452, and 1453, respectively.

[0083] 5. PON Protocol Architecture: Reservation Ethernet

[0084] One possibility for a protocol architecture for PON is the use ofa reservation ethernet approach. Here, a gating transmission is used,based on a request grant mechanism on top of ethernet. This approach isillustrated in FIG. 9, beginning with step 905. As in a DOCSIS-likeprotocol, the ethernet switch generates a map message or grant in step910, to indicate to the remotes when to transmit. A remote receives thegrants and determines, in step 915, whether transmission can take place.If not, the remote uses a gating mechanism in step 925 to hold theethernet transmission since the remote is not allowed to transmit.Otherwise, in step 920, the remote sends the transmission during thegranted periods. The process concludes at step 930. Hence this mechanismarbitrates access between remotes, but keeps the underlying ethernetframing transmission. The additional functionality required at theethernet switch is the gathering and scheduling of requests, creation ofacknowledgment responses, creation of map messages, and transmission ofthe messages downstream. The functionality required at the remote is thereception and interpretation of the map messages, creation of requests,and the gating mechanism to open or block the ethernet transmission. Thecontrol messages (such as grants and acknowledgments) generated by theethernet switch can be specified as new ethernet control messages. To befully ethernet compliant and avoid fragmentation of frames, minimumgrant size can be of a size to fit a payload equal to a minimum ethernetframe size (64 bytes). The message-carrying requests can also be definedto be of a size equal to this minimum frame size. Since request messagesare small, this message can be specified to allow the carrying of morethan one request at a time.

[0085] This gating mechanism based on grant messages from the CO (via,e.g., an OLT) to the ON (e.g., ONU) defines a basic communicationbetween the two. Once the CO recognizes the ON, this mechanism assigns aminimum amount of bandwidth to each ON. Additionally, the ON can requestmore bandwidth as needed. Hence, this mechanism has a contentionapproach only when the ON is recognized in the system. After this, theaccess of recognized ONs is contention free. The amount of bandwidthassigned to a recognized ON can be set at a fixed level when the ON isrecognized. This amount can be different for each ON depending on theservice agreement given to the ON. In addition, the ability to modifythis agreement can be defined in order to allow modification of servicesmore dynamically than just during registration time.

[0086] Another option for the PON protocol architecture is the use ofreservation aloha (request grant mechanism) as the underlyingtransmission mechanism. The protocol can be defined as a simple versionof DOCSIS with the minimum features in it. For example, fragmentation,payload header suppression, and downstream MPEG transport can beeliminated.

[0087] Possible relationships between PON arbitration and the 802.3protocol are illustrated in FIG. 10A and 10B. FIG. 10A illustrates thereservation ethernet case, discussed above, according to one embodimentof the invention. Here a PON arbitration process 1005 providestransmission control inputs 1010 to the 802.3 protocol. These inputs canbe translated to ethernet frames. And hence, the definition of thisprotocol just reduces to define the new frame types to carry thisadditional arbitration information. Alternatively, the 802.3 protocolcould be modified to incorporate the PON arbitration in a lower layer ofthe protocol stack as shown in FIG. 10B. A DOCSIS approach would definethe PON arbitration as an additional encapsulation mechanism. Anotherapproach is to consider the PON arbitration as physical layer signaling(such as invalid PCS codes in ethernet).

[0088]FIG. 10B illustrates the protocol relationships in the reservationaloha case, discussed above. Here, PON arbitration 1005 is below the802.3 protocol, which in turn is below IP layer 1020. Therefore, FIG.10B illustrates an architectural definition of a DOCSIS PON (DPON),wherein an additional header or protocol is provided for PONarbitration. On the other hand, FIG. 10A illustrates an architecturaldefinition for an ethernet-based PON (EPON), wherein the ethernetprotocol is extended to provide PON arbitration.

[0089] 6. Limiting PDU Size and Controlling Fragmentation

[0090] Under DOCSIS, when a remote receives a grant, it transmitspackets in its queue. In DOCSIS there is a one-to-one mapping betweenthe grant and the request. Hence the transmitted packets correspond tothe granted bandwidth except for a small amount of bandwidth due to theminislot-to-grant granularity.

[0091] This one-to-one mapping is not available if more smart mechanismsare available in the system. For example, the headend may generateadditional unsolicited grants. If a flexible use of grants isimplemented, any “flow” can use any grant independently of which flowgenerated the request. In this case, the granted bandwidth can be filledup with packets until no more packets fit. At the end of the burst itwill leave a space that may not fit the next packet to be transmitted.

[0092] There are generally two options. First, the space can be leftunused. This is inefficient. Second, the next packet could befragmented. Therefore, a system with no fragmentation may be inefficientif the burst lengths are not large enough. On average there is a wasteof half of a packet of average size, per burst. Depending on the burstand average packet sizes, this can be a significant waste.

[0093] An alternative to the customary fragmentation approach is tocoordinate the packetization at a higher protocol level and specify thesizes of unsolicited grants. In other words a maximum data unit (MDU)can be imposed to break the transmitted data into units that can bebetter handled in the system. This process is illustrated in FIG. 15.The process begins at step 1510. In step 1520, an MDU is determined tohave a size equal to M bytes. In step 1530, the amount of bandwidth tobe granted is defined, as an integer multiple of the MDU size. In step1540, a grant is issued, specifying bandwidth equal to K times the MDUsize. The process concludes at step 1550. By carefully choosing the sizeof unsolicited grants and the MDU size (e.g., defining the size of thegrant to be a multiple of the MDU), wasted bandwidth can be minimized.In an embodiment of the invention, different remotes in the system (andeven different flows) can operate with different MDU values.

[0094] Another alternative is to perform fragmentation in a moreflexible manner, depending on the bandwidth available. Again, what istypically done at the media access layer is now done at the IP layer. Inthis alternative, an arriving grant is examined to identify its size. AnIP packet is then fragmented so as to fit the grant, and the IP headeris modified as necessary. This is illustrated in FIG. 16. The processbegins at step 1610. In step 1620, a remote device receives a grant ofbandwidth. In step 1630, the remote device determines the size of thegranted bandwidth. In step 1640, a determination is made as to theamount of granted bandwidth that can be used by whole IP packets. Thisstep determines the number of IP packets that can be contained in thegranted bandwidth, and calculates the amount of bandwidth that isconsumed thereby. In step 1650, the remaining bandwidth is determined.In step 1660, the next IP packet is fragmented so as to use theremaining bandwidth. The process concludes at step 1670. In anembodiment of the invention, this adaptive process is implemented inhardware, and can be performed in real time.

[0095] 7. Using Video Bandwidth for Data

[0096] As is apparent from the above discussion, the proper allocationof bandwidth is required to service a set of users that have a varietyof needs. Ideally, allocation of bandwidth is flexible to allowservicing of different needs as they arise.

[0097] Digital video represents a large amount of data transmitted in acontinual stream, and therefore requires significant bandwidth.Accordingly, digital transmissions are generally allocated large amountsof bandwidth by default. But, under some circumstances, requirements fornon-video data may be great enough to exceed the default allocations forsuch data. In this case, bandwidth can be taken from transmissions suchas video, and reallocated to data channels that require more bandwidth.This requires monitoring of the demand for non-video data. If apredefined demand threshold is exceeded for non-video data, reallocationtakes place. If and when such demand returns to a predefined lowerlevel, the system can return to its default bandwidth allocations. In anembodiment of the invention, the reallocation of video bandwidth forother data transmissions can also depend on whether the demand for videois sufficiently low. This process is illustrated in FIG. 17. The processbegins with step 1710. In step 1720, a determination is made as towhether the current requirement for non-video bandwidth exceeds adefault value. If not, the normal default allocation for non-videobandwidth is used in step 1730. If the requirement for non-videobandwidth exceeds the default value, however, the process continues atstep 1740. Here, a determination is made as to whether the demand forvideo bandwidth is sufficiently low so as to permit reallocation ofvideo bandwidth to non-video data. If the demand for video is notsufficiently low, then the process continues at step 1730, and thenormal allocation of non-video data bandwidth is used. If, however, thedemand for video is sufficiently low to allow reallocation, then theprocess continues at step 1750. Here, video bandwidth is reallocated fornon-video data. The process then returns to step 1720 for continuedmonitoring of the requirement for non-video bandwidth.

[0098] 8. Allocating Bandwidth With Requests and Grants

[0099] In an embodiment of the invention, bandwidth can be allocatedflexibly by using a request/grant mechanism. Such an arrangement iscurrently defined in the DOCSIS 1.1 standard, but the concept can beadapted to a non-DOCSIS system as well. In such an arrangement, a remotedevice seeking to transmit does so after requesting bandwidth from acentral authority, such as a headend or similar module. If bandwidth isavailable, a grant is made by the central authority to the remote,specifying the bandwidth to be used by the remote (e.g., a specific timeinterval). This process is illustrated generally in FIG. 18. The processbegins with step 1810. In step 1820, a remote device requests bandwidthfrom the headend. In step 1830, a determination is made by the headendas to whether bandwidth is available. If not, a subsequent request forbandwidth can be made in step 1820. If bandwidth is available, then theprocess continues at step 1840, where the request for bandwidth isgranted, and the amount of bandwidth is specified in the grant. Theprocess concludes at step 1850.

[0100] In an embodiment of the invention, some or all grants can beunsolicited. During registration, bandwidth is allocated according to afixed assignment policy. As such, the headend can make unsolicited,fixed bandwidth allocations based on state for each remote device. Whenadditional remote devices register, the headend assigns the bandwidthallocation based on availability. In embodiments, the headenddynamically adjusts the bandwidth allocations as the system conditionschange, such as remote devices terminating or initiating sessions. TheCO keeps the state of the ON bandwidth needs based on the establishedsessions. In embodiments, the headend dynamically adjusts the bandwidthallocations in response to requests. The adjustment can be in accordancewith established dynamic service level agreements with the remotedevices.

[0101] Contention among remotes for granted bandwidth can be resolvedthrough a priority system or other mechanism. Note that in a TDMAcontext, the remote and headend must share the same sense of time. Thisallows a remote's sense of a granted timeslot (starting and endingpoints) to match that of the headend. Hence a synchronization processmay be required prior to any actual request/grant processing.

[0102] 9. Re-prioritization of Packets to Use Available Bandwidth

[0103] In some communications systems, a priority system is in place toresolve contention for available bandwidth. A packet having the highestpriority will generally be allowed to use the bandwidth, instead ofother lower priority packets that may need to be sent. In somesituations, however, this can be an inefficient arrangement. The highestpriority packet may be larger than the amount of available bandwidth.The priority logic dictates that only the highest priority packet can besent, yet this packet cannot be sent because of its size. In this case,the available bandwidth may go wasted.

[0104] To address this, an exception can be made to the normal priorityrules. If a lower priority packet will fit the available bandwidth, thispacket will be sent instead of the higher priority packet, rather thanwasting the bandwidth. In an embodiment of the invention, the packet tobe sent can be identified by choosing the highest priority packet amongthose that fit the available bandwidth. This is illustrated in FIG. 19.The process begins with step 1910. In step 1920, a remote devicereceives a bandwidth allocation. In step 1930, the remote deviceidentifies the highest-priority packet among the packets that need to besent. In step 1940, a determination is made as to whether the highestpriority packet fits the allocated bandwidth. If not, then in step 1950,the highest-priority packet is withdrawn from consideration, since itwould not fit the allocated bandwidth. The process would then continueat step 1930, where, among the remaining packets, the highest-prioritypacket is identified. If, in step 1940, the highest-priority packet fitsthe allocated bandwidth, then the process continues at step 1960. Here,the packet is sent. The process concludes at step 1970. Alternatively,if efficient bandwidth usage is important, the system can choose thelargest packet that will fit. Alternatively, some combination of bestfit and highest priority can be used to determine the packet to be sent.

[0105] III. User Services

[0106] A. Video Switching at Optical Node

[0107] Users typically desire the ability to readily control whatinformation they access. In the context of downstream digital video,this includes the ability to select a channel for viewing. Currentarchitectures provide for switching at a hub, such as hub 305 of FIG. 3.In response to a user command, hub 305 performs the requested switchingand forwards the appropriate transmission to ON 310, and ultimately tothe user.

[0108] This creates latency in system response to the user's commands,however, given that the command must go all the way to the hub 305,which must then react. Alternatively, the link 315 carries broadcastvideo of all transmissions to ON 310. Switching is then performed there,instead of at hub 305. While this requires greater bandwidth between hub305 and ON 310, the latency of the response to user input is reduced.Moreover, this switching function can also be performed at a centraloffice if, for example, the system does not include an optical node.

[0109] B. MPEG Buffering at Optical Node

[0110] When MPEG-formatted video is transmitted, a sequence ofindividual frames is organized into a “group of pictures” (GOP). A GOPbegins with an I frame, and is followed by B frames (or T frames,depending on the method of coding). Generally, if a user switches to atransmission at a time when a GOP has already started, i.e., after the Iframe, the entire GOP associated with that I frame is inaccessible.

[0111] This can be remedied if GOPs are buffered. This is illustrated inFIG. 20. Here, headend 2010 transmits a GOP 2020. GOP 2020 is held inbuffer 2040, located in optical node 2030. This makes each frame of GOP2020 available to user 2050. A user switching to a transmission inmid-GOP can then access a full GOP, starting with its I frame. In anembodiment of the invention, the GOP is buffered at the ON in a circularbuffer. When a user switches to a video transmission, he or she hasaccess to all of the current GOP, since all the GOP's frames up to thispoint, starting with the GOP's I frame, are available.

[0112] This concept can also be applied in contexts other than opticalnetworks. In general, buffering of video frames at an intermediate node,as described above, can take place in any access network having switchedvideo service. Moreover, buffering can also take place at a centraloffice when, for example, the system topology does not include an ON.

[0113] C. Channel Surfing and Proactive Streaming

[0114] Given the latency that can occur when a user switches amongdifferent video transmissions, the practice of scanning multipletransmissions in sequence (analogous to “channel surfing”) becomesdifficult. This can be addressed by making the switching functionalitymore intelligent. If, for example, switching is done at the ON (asdescribed above), the ON can be made to sense when channel surfing istaking place.

[0115] This is illustrated in FIG. 21. This process begins at step 2105.In step 2110, a determination is made as to whether a user has requestedsome number N of sequential switches within a predetermined window oftime. If so, it is assumed that the user is surfing and, in step 2115,the ON proactively sends the next transmission to the user prior toreceipt of the actual switch request. The determination of step 2110 canthen be repeated, and the next transmission can likewise be sentproactively, etc. When, in step 2110, it is determined that surfing hasstopped (i.e., fewer than N sequential switches within the time window),the next transmission is not sent, and the determination of step 2110 isrepeated. This serves to monitor the user for subsequent channelsurfing.

[0116] This concept can also be applied in contexts other than opticalnetworks. In general, detection of sequential switches and anticipationof future switching at an intermediate node, as described above, cantake place in any access network having switched video service.Moreover, this functionality can also be placed in a central officewhen, for example, the system does not include an ON.

[0117] B. Other DOCSIS Variations

[0118] Other variations on DOCSIS 1.1 can be used for the sake ofeconomy and computational simplicity. In particular, DOCSIS can beimplemented without one or more of the features specified by thestandard. For example, packet fragmentation/reconstruction and payloadheader suppression can be omitted, since these functions can becomputationally intensive. Likewise, the packet classification functioncan be limited. These omissions can make processing faster and can insome circumstances increase available bandwidth.

[0119] IV. Conclusion

[0120] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example, and not limitation. It will be apparent to personsskilled in the relevant art that various changes in detail can be madetherein without departing from the spirit and scope of the invention.Thus the present invention should not be limited by any of theabove-described exemplary embodiments.

What is claimed is:
 1. A fiberoptic, access communications system,comprising: a central office; and at least one end-user device inoptical communication with said central office, wherein said centraloffice comprises a physical layer device and a media access controller,wherein said media access controller comprises a gigabitmedia-independent interface that monitors incoming data from said atleast one end-user device to detect a field having a known relationshipto the start of a frame, for purposes of determining a ranging offset.2. The system of claim 1, wherein said system is an ethernet-basedcommunications system.
 3. The system of claim 1, wherein said system isa time division multiple access (TDMA) communications system.
 4. Afiberoptic, access communications system, comprising: a central office;at least one end-user device in optical communication with said centraloffice; and an aggregating optical node disposed between and incommunication with said central office, wherein said optical nodecomprises a physical layer device and a media access controller, andwherein said media access controller comprises a gigabitmedia-independent interface that monitors incoming data from said atleast one end-user device to detect a field having a known relationshipto the start of a frame, for purposes of determining a ranging offset.5. The system of claim 4, wherein said system is an ethernet-basedcommunications system.
 6. The system of claim 4, wherein said system isa time division multiple access (TDMA) communications system.
 7. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps performed at a headend device: a. phase locking atransmit bit rate to a clock of a headend counter; and b. transmittingdata, at the transmit bit rate, to an end-user device.
 8. A method ofoptimizing bandwidth efficiency in a fiberoptic ethernet-based timedivision multiple access communications system, comprising the followingsteps performed at an end-user device: a. receiving data at a transmitbit rate that has been phase-locked to a clock of a headend counter; andb. deriving a local clock on the basis of the received data.
 9. A methodof optimizing bandwidth efficiency in a fiberoptic ethernet-based timedivision multiple access communications system, comprising the followingsteps: a. receiving transmitted data; b. reading a synchronizationpattern at each of a plurality of points in the transmitted data; and c.when a threshold number of synchronization patterns have been readsuccessfully in succession, attaining synchronization.
 10. The method ofclaim 9, wherein the transmitted data comprises a plurality of MPEGframes, and each of the plurality of points in the transmitted data isthe initial byte of one of the MPEG frames.
 11. The method of claim 9,wherein the transmitted data comprises a plurality of variable-lengthpackets, and each of the plurality of points in the transmitted data isindicated by a pointer in a previous variable-length packet.
 12. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps, performed at an end-user device: a. receiving a grantmessage from an ethernet switch, wherein the grant message indicateswhen the end-user device may transmit; b. transmitting data at theindicated time; and c. withholding transmission of data otherwise.
 13. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communication system, comprising thefollowing steps: a. determining, at an internet protocol layer, anamount of bandwidth to be granted in an unsolicited grant where theamount is an integer multiple of a maximum data unit; and b. issuing theunsolicited grant.
 14. A method of optimizing bandwidth efficiency in afiberoptic ethernet-based time division multiple access communicationssystem, comprising the following steps: a. receiving an unsolicitedgrant of bandwidth; b. determining the amount of granted bandwidth; c.determining the amount of granted bandwidth that can be used by one ormore whole internet protocol packets; d. determining remainingbandwidth; and e. fragmenting a subsequent internet protocol packet suchthat a fragment of the subsequent internet protocol packet fits theremaining bandwidth.
 15. A method of optimizing bandwidth efficiency ina fiberoptic ethernet-based time division multiple access communicationssystem, comprising the following steps performed at a headend device: a.receiving a request for bandwidth; b. if the requested bandwidth isavailable, granting the request.
 16. A method of optimizing bandwidthefficiency in a fiberoptic ethernet-based time division multiple accesscommunications system, comprising the following steps performed at anend-user device: a. receiving an allocation of bandwidth; b. identifyinga highest priority packet to be transmitted; c. determining whether thehighest priority packet fits the allocation of bandwidth; d. if so,sending the highest priority packet; and e. if not, i) withdrawing thehighest priority packet from consideration; ii) identifying the nexthighest priority packet to be transmitted; and iii) determining whetherthe next highest priority packet fits the allocation of bandwidth.
 17. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps: a. registering an optical node; and b. allocatingbandwidth to the optical node based on a fixed assignment policy.
 18. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps: a. registering an optical node; b. assigning a fixedamount of bandwidth based on state to the optical node.
 19. A method ofoptimizing bandwidth efficiency in a fiberoptic ethernet-based timedivision multiple access communications system, comprising the followingsteps: a. allocating bandwidth to one or more optical nodes; and b.dynamically adjusting bandwidth allocations in response to changes instate.
 20. A method of optimizing bandwidth efficiency in a fiberopticethernet-based time division multiple access communications system,comprising the following steps: a. allocating bandwidth to one or moreoptical nodes; and b. dynamically adjusting bandwidth allocations inresponse to a request, wherein said adjusting is implemented inaccordance with an established dynamic service level agreement.
 21. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps: a. receiving an allocation of bandwidth; and b. sendinga packet as to fit the allocation of bandwidth.
 22. A method ofoptimizing bandwidth efficiency in a fiberoptic ethernet-based timedivision multiple access communications system, comprising the followingsteps: a. sending a request for bandwidth; b. receiving a grant; and c.sending a packet fitting the grant.
 23. The method of claim 22, furthercomprising the step of: receiving the grant in an Ethernet frame.
 24. Amethod of optimizing bandwidth efficiency in a fiberoptic ethernet-basedtime division multiple access communications system, comprising thefollowing steps: a. extending DOCSIS to govern transmissions within anoptical system; and b. allocating bandwidth to one or more optical nodesin accordance with DOCSIS.
 25. The method of claim 24, wherein step (a)comprises: providing a physical medium responsive to sending and/orreceiving DOCSIS-compliant communications within an optical system. 26.The method of claim 24, wherein step (a) comprises: executing subscriberservice functions from a hub in communication with an optical node. 27.The method of claim 24, wherein step (a) comprises: operating an opticalnode to allocate bandwidth.
 28. The method of claim 24, wherein step (a)comprises: operating an optical node to implement timing functions. 29.The method of claim 24, wherein step (a) comprises: implementing arequest-grant mechanism to allocate bandwidth.
 30. The method of claim24, wherein step (a) comprises: not using fragmentation duringtransmissions within the optical system.
 31. The method of claim 24,wherein step (a) comprises: not using payload header suppression duringtransmissions within the optical system.
 32. The method of claim 24,wherein step (a) comprises: simplifying the classification processduring transmissions within the optical system.
 33. The method of claim24, wherein step (b) is based on an unsolicited grant service.
 34. Themethod of claim 33, wherein step (b) comprises: extending theunsolicited grant service to accept and/or respond to additionalrequests.
 35. A method of optimizing bandwidth efficiency in afiberoptic ethernet-based time division multiple access communicationssystem, comprising the following steps performed at an end-user device:a. receiving an allocation of bandwidth; b. identifying a highestpriority packet to be transmitted; c. determining whether the highestpriority packet fits the allocation of bandwidth; d. if so, sending thehighest priority packet; and e. if not, i) withdrawing the highestpriority packet from consideration; ii) identifying the first packet ofthe next priority level to be transmitted; and iii) determining whetherthe next highest priority packet fits the allocation of bandwidth.